1. Field of the Invention
This invention relates generally to a pick-up feeding or moving mechanism for use with a servo drive apparatus and more particularly to a drive circuit and a moving apparatus in which a mechanism for stably feeding a pick-up at high or low speed is formed of a differential transmission system.
2. Description of the Prior Art
A variable range in which a playback optical pick-up used in an optical disc reproducing apparatus or the like can be moved is very large. On the other hand, the track pitch between the tracks on the optical disc is selected to be about 1.6 .mu.m, and in a so-called compact disc (CD) and so on, the revolution speed thereof is low, and the feeding pitch or length of the optical pick-up is very slow such as about 10 .mu.m per second. Therefore, when the pick-up is stably driven by a motor, the deceleration ratio thereof must be selected to be large. On the other hand, when a desired position on the disc is accessed at high speed, the pick-up must be moved at high speed. For example, when the pick-up is moved at, for example, 10 cm per second, this speed has a difference of 10.sup.4 times that upon the playback mode. To this end, in order to make a normal playback stable, the slow response must be made and hence the deceleration ratio must be selected large. If the deceleration ratio is small, the movement of the pick-up becomes discontinuous due to the uneven actuation of a drive motor and an initial actuation voltage is needed until the motor begins to rotate. Further, due to the change from the static friction to the dynamic friction of the motor, there is caused a hysteresis characteristic and the like, so that it is difficult to carry out the slow servo. On the contrary, when the motor is decelerated in response to the high speed feeding, the motor rotation becomes very low in the normal playback mode so that the driving voltage becomes substantially equal to or less than the actuation voltage. Thus, the motor is repeatedly stopped and actuated and thereby driven unstably.
In order to remove such problems, the prior art transmission system is constructed as shown in FIG. 1. Referring to FIG. 1, in the normal playback mode, a motor M2 is provided with a large deceleration gear G and rotates, through a clutch C2, a screw 1, which rotates an optical pick-up PU, so that the motor M2 is rotated in the system formed of an input voltage VE, a movable contact a of a switch S, its fixed terminal c, a driving amplifier A2 and the motor M2, while the clutch C2 is operated by the system formed of a voltage supply source E, another movable contact a.sub.1 of the switch S, its fixed contact c.sub.1 and the clutch C2 to thereby move the optical pick-up PU as a load. Whereas, in the case of high speed mode, the switch S is changed in position, the clutch C2 is made inoperable and a motor M1 is driven through a driving amplifier A1 by the input voltage VE so that the screw 1 can be rotated at high speed and hence the optical pick-up PU can be moved at high speed. When switchably changing the high speed and low speed, the mechanical clutch C2 is required and this makes the moving apparatus complicated in construction. Further, the clutch C2 produces an operation noise and the like, furthermore, the accuracy and precision upon assembly must be increased.